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Mark Hedley and M. K. Yau

Abstract

A three-dimensional anelastic model is used to perform simulations of explosive cyclones. The major goals are 1) to investigate the importance of horizontal resolution on the simulation of mesoscale features during explosive cyclogenesis using a very high resolution nonhydrostatic model and 2) to determine the impact of the destabilization of the lower troposphere by strong surface fluxes on the development of the storm.

The results from six experiments are presented. The main conclusions are:

1) By using analytic initial conditions based on typical wintertime conditions prior to explosive cyclogenesis, it is possible to obtain very realistic simulations of rapid cyclogenesis.

2) The use of high horizontal resolution is important in simulating the mesoscale features of rapidly deepening cyclones. In particular, the structure of the intense warm front observed ahead of the cyclone is very sensitive to changes in horizontal resolution.

3) The destabilization of the lower troposphere prior to the period of rapid deepening is essential in the formation of an extremely intense frontal structure, which in turn is instrumental in the rapid spinup of the storm.

4) In the presence of strong surface fluxes, the development of the simulated cyclone is affected by the depth of the planetary boundary layer.

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G. Balasubramanian and M. K. Yau

Abstract

The life cycle of an intense marine cyclone is documented in this paper. The departure of the moist dynamics from the dry baroclinic dynamics is explored from an energetics point of view. The contributions of various physical processes through the life cycle to the low-level cyclonic circulation is computed using a recently developed PV (potential vorticity) inversion technique.

The moist cyclone deviates most from the dry cyclone during the early rapid spinup period with significant mesoscale features associated with the warm and bent-back warm frontal zones. However, from an energetics point of view, the moist cyclone possesses a very similar, but enhanced, growth and decay rate during its life cycle. The transports of heat and momentum fluxes are also strengthened. The enhancement of eddy kinetic energy due to condensation accounts for nearly 50% of the maximum eddy kinetic energy generated in the moist cyclone.

From a PV perspective, the main difference between the moist cyclone and the dry cyclone is the production of a low-level PV anomaly during the early rapid spinup period. The cold advection in association with the circulation due to this anomaly has the cyclolytic effect of decreasing the surface thermal anomaly and the cyclogenetic effect of increasing the upper-level wave deepening. In the mature stage when the growth has almost ceased, the dry cyclone also possesses upper- and lower-level PV anomalies very similar to the moist cyclone.

Based on these results, the authors conclude that, except for the mesoscale structural differences and their associated interactions during the early rapid spinup period, the moist cyclone exhibits an enhanced growth rate (and decay rate as well) but appears dynamically similar to the dry cyclone from an energetics point of view as well as in terms of “PV thinldng.”

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G. Balasubramanian and M. K. Yau

Abstract

A hydrostatic, primitive equation model is used to simulate an oceanic cyclone with idealized initial conditions. The model uses a pressure coordinate in the vertical with a grid spacing of 100 mb. In the horizontal a grid spacing of 25 km is used, which should be nearly sufficient to resolve slantwise convection. The model produces an explosive moist cyclone with an intense bent-back warm front. The thermal gradient in the bent-back warm frontal region exceeds 8 K/100 km, in agreement with recent observations.

Before rapid deepening, the model atmosphere becomes unstable to slantwise convection in the warm frontal region. After the spinup period, buckling in the angular momentum and θε surfaces are noted. It is suggested that the descending motion and the associated dry slot over the cyclone center may arise from the descending branch of the slantwise convection on the warm side of the warm front. The descent may be augmented by the evaporation of liquid water. After the explosive deepening period, the stratification in both the warm front and the bent-back warm front exhibits neutrality to slantwise convection.

The Ertel potential vorticity (EPV) inversion technique developed by Davis and Emanuel is used to obtain the perturbation geopotential at 900-mb, 500-mb, and 300-mb levels due to EPV anomalies at different levels. The inversion is applied at the mature stage of the cyclone at 45 h. It is found that there is a positive EPV anomaly along the regions of the warm front and bent-back warm front, and it accounts for 40% of the perturbation geopotential at 900 and 500 mb over the cyclone center. The contribution of low-level EPV anomaly in the moist cyclone to the perturbation geopotential at 500 mb over the cyclone center is twice that in the dry case. The circulation of the inverted nondivergent wind fields in the moist run shows a small-scale cyclonic vortex and the presence of cold advection in the bent-back warm frontal region.

The contribution of the upper-level EPV anomaly to the 900-mb perturbation geopotential is also significant in the moist cyclone. The physical mechanism for the latter effect can be traced to an increase in vorticity advection in the middle troposphere in association with the formation of the bent-back warm front. This finding is in agreement with the authors' recent two-layer model results, which show that the bent-back warm front represents a region of cold advection that can in turn lead to an intensification of the upper-level wave. The contribution of the 1000-mb θ anomaly in the mature moist cyclone is smaller than that of the dry run because of the convection-induced cold advection in the bent-back warm front.

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Yongsheng Chen and M. K. Yau

Abstract

Highly asymmetric structures in a landfalling hurricane can lead to the formation of heavy rains, wind gusts, and tornados at prefered locations relative to the center of the hurricane. In this study, the development of asymmetric structures in an explicitly simulated idealized hurricane during landfall was investigated.

It was found that the boundary layer friction and its associated convection produce a low-level positive potential vorticity (PV) band ahead of the hurricane. The interaction between the PV band and the eyewall PV ring leads to a temporary weakening and reintensifying cycle. Asymmetric structures arise from the near discontinuity of the surface friction and the latent heat flux. The breaking of the eyewall in the rear quadrants is favorable for the intrusion of the low moist entropy air into the core. Consequently, PV increases significantly in the core, in and just above the boundary layer due to the stabilization. After the hurricane makes landfall, the diabatic heating in the eyewall is reduced and cannot generate enough PV to maintain the PV ring in the middle and upper troposphere. The PV ring evolves into a monopolar structure through the nonlinear mixing process.

The Eliassen–Palm (EP) flux and its divergence in the Eulerian mean equations in isentropic coordinates are applied to explore the wave dynamics and wave–mean flow interactions. The vortex Rossby wave–related eddy momentum and heat transports, indicated by the EP flux, vary as a response to the evolution of the PV structure. The wave–mean flow interaction has a significant effect on the tangential wind, which is dominated by the mean circulation, especially the symmetric diabatic heating. Together with the asymmetric diabatic heating, the waves tend to counteract the effect of the mean circulation.

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G. Balasubramanian and M. K. Yau

Abstract

A two-layer primitive equation model with a parameterization for slantwise convection is used to study explosive cyclones. The parameterization includes a simple representation of the planetary boundary layer, as well as deep and shallow cloud types. The major results are 1) the two-layer model produces a realistic explosive cyclone, 2) the explosive deepening coincides with the formation of the “bent-back warm front,” 3) fronto-genetic calculations reveal an indirect circulation due to “tilting” in the intense warm front, and 4) the quasi-Lagrangian heat and vorticity budgets indicate strong interaction of the lower- and upper-level flow during the rapid spinup stage.

These results suggest a sequence of events involved in some explosive cyclogenesis: Convection leads to rapid frontogenesis and the formation of a bent-back warm front. The sudden surge of cold advection in the regions of the bent-back warm front then forces the upper-level heights over the cyclone center to fall in a rather dramatic way. Increased upper-level vorticity advection interacts with the low-level system, leading to explosive cyclogenesis.

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Majid Fekri and M. K. Yau

Abstract

This study presents an information-theoretical score (ITS) with an emphasis on desirable and undesirable mutual information between a series of dichotomous forecast and observation. As ITS makes use of the same contingency table as traditional scores, the performance of threat score (TS), equitable threat score (ETS), and true skill statistics (TSS) are compared with ITS using three different approaches. First, a hypothetical forecast setup is employed to investigate the responses of the scores to bias, phase error, and event frequency. It was found that the desirable mutual information portion of ITS (C +) is closer to TSS, and the undesirable mutual information portion of ITS (C ) reveals the presence of biases and random errors in the forecast. There is also a similarity between ITS and ETS. Second, the sensitivities of ITS and ETS to forecast bias tendency are examined analytically using the critical performance ratio (CPR). It is shown that ITS has a more dynamical response to incremental bias. By increasing the bias, the CPR value of ITS increases more rapidly than that of ETS indicating a higher resistance to hedging. Third, the skill scores on two sets of operational forecasts are applied with respect to a mosaic of observed radar reflectivity. The results show that ITS remains more consistent in its evaluation of skills at different thresholds compared to other scores.

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Mark Hedley and M. K. Yau

Abstract

A two-dimensional anelastic model is used to study the propagation of errors arising from the use of open lateral boundaries. Reference experiments were performed using very large horizontal domains. Other experiments were carried out in smaller domains with various radiation boundary conditions. Direct calculation of the error fields demonstrates the mechanism for the generation, propagation, and growth of errors. It was shown that a fixed phase speed method resulted in runaway circulation while a floating speed scheme exhibited difficulties in regions where horizontal gradients in horizontal velocity are small. A hybrid scheme is proposed which was shown to work well in highly truncated spatial domains.

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Konstantinos Menelaou and M. K. Yau

Abstract

The role of asymmetric convection to the intensity change of a weak vortex is investigated with the aid of a “dry” thermally forced model. Numerical experiments are conducted, starting with a weak vortex forced by a localized thermal anomaly. The concept of wave activity, the Eliassen–Palm flux, and eddy kinetic energy are then applied to identify the nature of the dominant generated waves and to diagnose their kinematics, structure, and impact on the primary vortex. The physical reasons for which disagreements with previous studies exist are also investigated utilizing the governing equation for potential vorticity (PV) perturbations and a number of sensitivity experiments.

From the control experiment, it is found that the response of the vortex is dominated by the radiation of a damped sheared vortex Rossby wave (VRW) that acts to accelerate the symmetric flow through the transport of angular momentum. An increase of the kinetic energy of the symmetric flow by the VRW is shown also from the eddy kinetic energy budget. Additional tests performed on the structure and the magnitude of the initial thermal forcing confirm the robustness of the results and emphasize the significance of the wave–mean flow interaction to the intensification process.

From the sensitivity experiments, it is found that for a localized thermal anomaly, regardless of the baroclinicity of the vortex and the radial and vertical gradients of the thermal forcing, the resultant PV perturbation follows a damping behavior, thus suggesting that deceleration of the vortex should not be expected.

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G. Balasubramanian and M. K. Yau

Abstract

A three-layer primitive equation model with a representation of slantwise convection is used to study explosive marine cyclogenesis. A simple representation of boundary layer and shallow and deep cloud types is considered. Apart from the control simulation. experiments are conducted to test the sensitivity of the bent-back warm front and the explosive deepening to various model parameters.

The major results are as follows: 1) The control simulation produces a realistic explosive cyclone. 2) The cloud mass fraction at the top of the boundary layer, which is equal to the fractional area of slantwise ascent in a grid box, essentially determines the cloud mass flux transported into the large-scale atmosphere. The thermal gradient in the bent-back warm frontal region and the final deepening strongly depend on this parameter. 3) Shallow clouds are relatively insignificant in affecting the dynamics of explosive cyclones. 4) The surface drag force weakens the development of strong horizontal wind shear and the bent-back warm front. 5) The thermal gradient in the bent-back warm front gradually increases with the thermodynamic disequilibrium between the sea surface and the atmospheric boundary layer. 6) Enhanced vertical wind shear increases the deepening rate. However, the asymmetry in the wave development and strong pressure falls near the warm front are associated with slantwise convection. 7) Weak low-level stability has significant impact on cyclogenesis. 8) The effect of stable condensation is to moderately accelerate the development of the baroclinic wave.

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Konstantinos Menelaou and M. K. Yau

Abstract

Although intense tropical cyclones (TCs) are considered to be axisymmetric vortices, observations reveal that they are often highly asymmetric. Better understanding of the underlying asymmetric dynamics is a critical step toward advancing TC intensity forecasting. In this paper, we revisit the mechanisms behind one of the most frequent asymmetric patterns: the deformation of the core into an elliptical shape. Previously, elliptical eyewalls were primarily thought to be an outcome of barotropic instability, a mechanism that involves the coupling and mutual growth of counterpropagating vortex Rossby (VR) waves. These results were largely based on simplified numerical models that filter out inertia–gravity (IG) waves. Consideration of IG waves introduces the possibility of an additional instability mechanism, one that involves a VR wave that spontaneously emits a spiral IG wave into the environment. We provide evidence that elliptical eyewalls, which may form within a three-dimensional primitive-equation nonlinear model that supports both instability types, can solely originate by the mechanism of spontaneous radiative imbalance. These evidences are supported by a number of nonlinear simulations, supplemental linear eigenmode analysis, and a linear simulation. The potential role of a multimechanistic instability is also briefly addressed.

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